1. Field of the Invention
The present invention relates to a luminous dial plate structure for watches, and more particularly to a luminous dial plate structure for watches using a phosphorescence-maintaining luminous agent, abundant in ornamental effect.
2. Related Background Art
Conventional luminous dial plates for watches are roughly classified into two types, i.e., spontaneous-emission-type luminous dial plates for professional diver's watches and luminous dial plates for general watches utilizing the phosphorescence. The former luminous dial plates for professional diver's watches are provided with a time indication scale which is a coating of radioactive substance such as, promethium or tritium, having such an advantage that the scale is visually recognizable for a long time in a dark environment. However, because the radioactive substances are hazardous, applications are limited only to very special purposes such as the luminous dial plates for professional diver's watches as described above.
Therefore, normal watches broadly use the luminous dial plates utilizing the phosphorescence of sulfur or sulfides, for example of phosphorus.
The conventional luminous dial plates for watches utilizing the phosphorescence of sulfides, however, had such problems that the luminance was relatively low and that an emission time, i.e., a visible time was relatively short, for example about one hour. Further, as for the ornamental effect which is an important factor for watch dial plates, there was no contribution of the phosphorescence, so that a demand has been high to develop a watch dial plate utilizing the phosphorescence also in decoration for watch dial plates.